1. Field of the Invention
The present invention relates to communication systems, and, in particular, to mobile telephony systems that conform, for example, to the IS-95 standard.
2. Description of the Related Art
A typical mobile telephony system has a distribution of base stations located throughout the system""s coverage area, with each base station supporting communications with those mobile units that are currently located within the coverage range of that base station. At the initiation of communications with a base station, each mobile unit derives a time reference from the arrival of either the forward-link pilot channel or the forward-link paging channel received from the base station (depending on whether the communications are initiated by the mobile unit or the base station). The base station derives its own time reference from global positioning system (GPS) signals received by a GPS receiver at the base station.
A mobile unit uses the derived time reference to control the timing of events within each processing cycle. For example, a mobile unit uses the derived time reference to determine when to transmit reverse-link communication signals back to the base station. The receiver at the base station uses its GPS time reference to define a search window in each signal period within which it scans for reverse-link signals transmitted by the mobile unit.
FIG. 1 shows a block diagram of a mobile unit 102 and a base station 104 of a mobile telephony system. One of the characteristics of a mobile telephony system is that signals transmitted from one node in the system (e.g., base station 104) may reach another node (e.g., mobile unit 102) by two or more different signal paths. For example, one signal path may be thexe2x80x9cdirectxe2x80x9d line-of-sight path 108 between the two nodes, while other signal paths may be xe2x80x9cindirectxe2x80x9d paths, such as path 110, in which signals are received by the second node after reflecting off one or more different physical objects (e.g., a mountain, a building, or even the atmosphere), such as physical object 106. Because the different signal paths will typically have different lengths, different versions of the same signal will be received at the second node at slightly different times. This phenomenon is referred to as multi-path and the different received versions of a single signal are referred to as the multi-path components. In an IS-95 communication system, each mobile unit selects its time reference based on the earliest arriving multi-path component. See Section 6.1.5.1 of the IS-95A Specification.
Another characteristic of a mobile telephony system is that the relative signal strengths of the individual components of a multi-path signal can vary over time. More often than not this results from movement of the mobile units within a base station coverage area. For example, as the user of a mobile unit drives in a car, the direct line-of-sight path between the base station and the mobile unit may become temporarily blocked by other vehicles, buildings, or terrain. During such times, the multi-path component corresponding to the direct line-of-sight path may become significantly attenuated. In that case, the direct line-of-sight multi-path component may no longer be the earliest arriving multi-path component of sufficient signal strength, and the mobile unit may need to derive a new time reference based on a different xe2x80x9cearliest arrivingxe2x80x9d multi-path component of the forward-link signal. Since this different multi-path component will follow a signal path from the base station to the mobile unit having a length longer than the direct line-of-sight signal path, the new derived time reference will be later than the previous derived time reference. Similarly, when the temporary obstruction ends, the direct line-of sight signal may once again become the earliest arriving multi-path component, and the mobile unit will again derive a new time reference, which in this case will be earlier than the immediately previous derived time reference, which was based on an indirect signal path.
Whenever the mobile unit changes its time reference, it will adjust the timing at which it transmits reverse-link signals to the base station. As such, the reverse-link signals will arrive at the receiver(s) of the base station at different times within the base station""s processing cycle.
The size of the search window used by a base station receiver to search for reverse-link signals transmitted by a mobile unit is typically based on the expected variations in the time of arrival of the reverse-link signals due primarily to changes in the multi-path signals. These variations are caused both by changes in the forward-link multi-path signals (from which the mobile unit derives its time reference) as well as by changes in the reverse-link signals, which can also have their own varying multi-path components. The range of these variations in arrival time of the reverse-link signal at the base station is referred to as the delay spread.
Although the size of the base station search window is typically selected to handle normal variations in the time of arrival of the reverse-link signals, there are situations that can occur for which the base station search window is typically too small. For example, when a mobile unit starts to hand off from an old base station to a new base station, the mobile unit may begin to treat the signal from the new base station as a component of the multi-path signal. If the signal from the new base station arrives earlier than any of the signal components from the old base station, the mobile unit may change its derived time reference based on the earlier arriving signal from the new base station. If the arrival time of the signal from the new base station is sufficiently earlier than the earliest arriving component from the old base station, the time reference may change so much that the reverse-link signals will arrive at the old base station before the start of it search window. In that case, the communication link between the mobile unit and the old base station will be dropped. The same thing will happen to the new base station. The result will be the complete termination of communications with the mobile unit.
One way to limit this problem is to increase the size of the base station search window to handle even larger variations in arrival time of the reverse-link signals. This is an undesirable solution because it greatly increases the routine processing demands in the base station just to handle a problem that occurs only in certain limited circumstances.
The present invention is directed to a signal search scheme for mobile telephony systems that addresses the problem of dropped communications when changes in the time reference used by a mobile unit to transmit reverse-link signals to a base station cause the received signal to fall outside of the base station search window. According to the present invention, the receiver in the base station uses two search windows to scan for reverse-link signals transmitted by a mobile unit: (1) a first search window and (2) a second search window. The timing of the second search window (i.e., the start and stop times) follows the detected signal, while the timing of the first search window remains relatively constant. If the timing of the detected signal remains relatively constant for a sufficient length of time (e.g., a specific number of processing cycles), then the timing of the first search window is adjusted based on the timing of the most recently received signal.
The present invention allows the base station receiver to continue to detect reverse-link signals, even when the derived time reference used by the mobile unit changes greatly. According to the present invention, the second search window at the base station receiver will track the received signals even when they fall outside of the first search window. At the same time, the search scheme of the present invention is designed to avoid moving the first search window during relatively short periods of noise or interference.
According to one embodiment of the present invention, a first search window and a second search window are defined, and scanning is performed for the received signal within both search windows. The timing of the second search window is adjusted to follow the received signal from cycle to cycle, while maintaining the timing of the first search window. The timing of the first search window is adjusted only when a specified criterion is met, e.g., when the timing of the detected signal remains relatively constant for a specified number of processing cycles.